Precisely wavelength-tunable and wavelength-switchable narrow linewidth lasers

ABSTRACT

Continuously tunable and precisely wavelength-switchable fiber lasers combine fiber Bragg gratings and the transmissive filtering properties of high finesse fiber Fabry-Perot filters. This laser arrangement adapts to multiple wavelength ranges based on the selections of fiber Bragg grating and gain medium and their arrangement to create a wavelength-modulatable and simultaneously rapidly wavelength-switchable narrow linewidth all-fiber laser design. This laser arrangement further results in narrow-linewidth outputs with fast switching speeds between the selected wavelengths.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application makes reference to co-pending U.S. Provisional PatentApplication No. 60/110,056, entitled “Novel Wavelength-Modulatable andContinuously Tunable Narrow-Linewidth Fiber Lasers and PreciselyWavelength-Switchable Narrow-Linewidth Laser for OpticalTelecommunications and Spectroscopic Applications,” filed Nov. 25, 1998,and U.S. patent application Ser. No. 09/246,125, entitled “Tunable BraggGratings and Devices Employing the Same,” filed Feb. 8, 1999, the entirecontents and disclosures of which are hereby incorporated by reference.

This invention is made with government support under contract numbersMDA-972-94-1-0003 and MDA-972-98-1-0002, awarded by DARPA. Thegovernment may have certain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to tunable fiber lasers.

2. Description of the Prior Art

Tunable lasers have applications in a wide variety of fields, includingoptical communications and spectroscopy. For trace gas monitoring,compact tunable (20-30 GHz), wavelength modulatable sources with outputpowers on the order of approximately 100 μW and linewidths better than100 MHz are in great demand for wavelength modulation spectroscopy. Theavailability of high performance erbium-doped fiber amplifiers andpumped lasers allows for tunable fiber lasers. Modulation and switchingof optical signals are basic functions in an optical communicationsystem. Through modulation, the information to be communicated isexpressed in one or more parameters of a light signal, such as theamplitude, the polarization, the phase or frequency of the field, or ofthe magnitude or spatial distribution of the power and/or intensity.Through switching, the light signal may be routed through a network ofoptical nodes and connections.

Precisely wavelength-switchable narrow linewidth laser sources are ofgreat interest for many photonic applications, such as for tuning “onand off” narrow absorption lines in spectroscopic measurements,including the monitoring of resonantly absorbing species in DIAL(differential absorption LIDAR)-type applications. Wavelength-switchablenarrow linewidth laser sources are also critically needed for severalapplications in WDM-based (wavelength division multiplexed) fiber opticcommunication systems. Key requirements for such multi-wavelengthswitchable sources for WDM/DWDM systems are: (1) an accurate match withthe wavelength channels on the WDM/DWDM ITU grid, (2) an arbitrary setof such channels, (3) a capability for switching reliably to any channelbetween such a preselected arbitrary set of channels, (4) low crosstalk,and (5) microsecond (or faster) switching speeds.

Past multi-wavelength switchable sources have in general been limited toschemes that are either difficult to scale to a large number ofwavelengths, or have relatively slow (millisecond) switching speeds.Laser arrangements such as those found in U.S. Pat. No. 5,504,771 alsorequire the use of stable external “wavelength lockers” to preventwavelength drift from the FFPs PZT tuning assembly. Multi-frequencylasers based on integrated-optic arrays of DBR and DFB lasers, or SOA(semiconductor optical amplifiers arrays integrated with AWGs (arrayedwaveguide gratings) seem to satisfy most of the above requirements.However, these are relatively difficult and expensive to manufacture,particularly in small volumes or for custom applications that mayrequire a combination of numerous arbitrarily-spaced channels on theWDM/DWDM ITU grid.

Therefore, there is still a need for a rapidly switchablemulti-wavelength source that is relatively easy to manufacture for anycustomized set of arbitrary channels on the ITU grid.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a rapidlyswitchable multi-wavelength source that is relatively easy tomanufacture for any customized set of arbitrary channels.

It is a further object to provide precise wavelength switching andselectivity capable of achieving specifications needed for DWDMapplications.

It is yet another object to provide continuously wavelength-tunablefiber lasers and rapidly and precisely wavelength-switchable fiberlasers.

It is yet another object to provide continuous wave output as well aspulsed laser source designs.

It is yet another object to provide for single-wavelength andmulti-wavelength switchable and tunable emission.

It is yet another object to prevent wavelength drift from the FFPs PZTtuning assembly by providing stable wavelength intra-cavity filtersthrough the use of fixed-wavelength fiber Bragg gratings.

It is yet another object to provide wavelength-modulatable andsimultaneously rapidly wavelength-switchable narrow linewidth all-fiberlaser design for ultra-sensitive detection of single or multiple tracegas species.

It is yet another object to provide fine electronic tuning over acoarsely selected wavelength range.

It is yet another object to provide narrow linewidth outputs, superiorpower outputs, and lower RIN (relative intensity noise) than elaborateshort cavity lasers.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in conjunction with the accompanyingdrawings, in which:

FIG. 1A is a schematic diagram of a Sagnac loop travelling wave laser;

FIG. 1B is a tuning curve for a travelling wave laser constructed inaccordance with FIG. 1A;

FIG. 2A is a schematic diagram of a standing wave laser;

FIG. 2B is a tuning curve for a standing wave laser constructed inaccordance with FIG. 2A;

FIG. 3 is a schematic drawing of a linear cavity standing wave laseraccording to a preferred embodiment of the invention;

FIG. 4 is a schematic drawing of a standing wane laser according to analternate embodiment of the invention;

FIG. 5 is a schematic diagram of a wavelength-selectable fiber laseraccording to an alternate embodiment of the invention;

FIG. 6 is a schematic drawing of a quasi-unidirectional travelling wavelaser according to yet another alternate embodiment of the invention;

FIG. 7 is a schematic drawing of a quasi-unidirectional travelling wavelaser according to yet another alternate embodiment of the invention;

FIG. 8 is a schematic drawing of wavelength-switchable and tunable fiberlaser using a three-port circulator according to yet another alternateembodiment of the present invention;

FIG. 9 is a schematic drawing of wavelength-switchable and tunable fiberlaser using a four-port circulator according to yet another alternateembodiment of the present invention;

FIG. 10 is a schematic drawing of wavelength-switchable and tunablefiber laser using a four-port circulator or two cascaded 3-portsaccording to yet another alternate embodiment of the present invention;

FIG. 11 is a schematic drawing of wavelength-switchable and tunablefiber laser using a three-port circulator according to yet anotheralternate embodiment of the present invention;

FIG. 12 is a diagram of the Channel Spacing of the laser of FIG. 5;

FIG. 13 is a diagram showing the FFP Free Spectral Range of the laser ofFIG. 5;

FIGS. 14A and 14B show the results of a two-channel switchingdemonstration and a spectrum range from 1524.50 nm to 1559.50 nm;

FIG. 15A shows the wavelength output of the laser configuration of FIG.5 with 33.6 volts applied to the PZT element of the FFP;

FIG. 15B shows the wavelength output of the laser configuration of FIG.5 with 34.1 volts applied to the PZT element of the FFP;

FIG. 16 illustrates a compact hybrid rapidly wavelength-switchable andwavelength-tunable continuous wave (CW) or pulsed source constructed inaccordance with yet another embodiment of the invention;

FIG. 17 illustrates a compact hybrid rapidly wavelength-switchable andwavelength-tunable continuous wave (CW) or pulsed source constructed inaccordance with yet another embodiment of the invention;

FIG. 18 illustrates a rapidly wavelength-switchable andwavelength-tunable continuous wave (CW) or pulsed source constructed inaccordance with yet another embodiment of the invention; and

FIG. 19 illustrates rapidly wavelength-switchable and wavelength-tunablecontinuous wave (CW) or pulsed source constructed in accordance with yetanother embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

It is advantageous to define several terms before describing theinvention. It should be appreciated that the following definitions areused throughout this application.

Definitions

Where the definition of terms departs from the commonly used meaning ofthe term, applicant intends to utilize the definitions provided below,unless specifically indicated otherwise.

The term “laser cavity resonator” for the purposes of the presentinvention refers to the device having a round trip path for the opticalfield of the laser. This propagation of the optical field in theroundtrip path may be facilitated by various means which include, butare not limited to, fiberoptic waveguides, planar waveguides in bulkglass, planar waveguides in semiconductors, as well as free space (i.e.open cavity resonators in MEMS-based designs).

The term “Bragg grating” or “fiber Bragg grating (FBG)”, for thepurposes of the present invention, refers to a structure containingalternating periodic segment arms of varying periods of high and lowrefractive index material segment arms and/or appropriately embeddedphase shift segment arms at well defined locations of the structure. A“period” for the purposes of the present invention is defined as one setof adjacent high and low refractive material segment arms. It isunderstood by this definition that the order of the high and low indexmaterials is irrelevant, only that there is a change in refractive indexbetween adjacent segment arms. While only uniform gratings areillustrated below, non-uniform gratings are also contemplated within thescope of the invention.

The term “waveguide” for the purposes of the present invention refers toany device used to channel an optical signal, at any frequency. Specificexamples of waveguides include, but are not limited to: fiber-opticwaveguides, planar glass, as well as crystalline and semiconductorwaveguides.

The term “input voltage” for the purposes of the present inventionrefers to any voltage that is applied to the devices discussed below. Inparticular embodiments, specific voltages are used. Examples of thesevoltages include, but are not limited to: a DC voltage, an AC voltage,and pulsed voltage.

The term “ITU grid” for the purposes of the present invention refers toa standard grid of WDM channels: discrete set of pre-designatedwavelengths that are: (1) used as optical carriers of information (i.e.,wavelength channels separated by 100 GHz in the ITU DWDM grid), or (2)used as signals for the control, generation, routing, and supervision ofthe above-mentioned optical carrier wavelength channels. Therelationship between wavelength_(carrier) and wavelength_(control) isillustrated in the case of wavelength conversion through FWM (four wavemixing) in semiconductor optical amplifiers. In this example, the pumpwavelengths are the control wavelengths i.e., wavelength_(pump), whilethe source and target wavelengths are the carrier wavelengths, i.e.,wavelength_(carrier).

The term “multiwavelenght multiwavelength grid filter” for the purposeof the present invention refers to a set of wavelengths that are equallyspaced or whose spacing are multiples of a common interval. The devicesets up a pre-designated set of wavelength channels. Examples of suchpre-designated sets include, but are not limited to, the 50 and 100 GHzWDM wavelength standards defined by the ITU, and strong near-infrared(1.5 um spectral region) absorption lines of chosen molecules. Thedevice may have single or multiple wavelength passbands. The device maybe composed of a single optical filter or a set of such optical filters.Specific examples of multi-wavelength grid filters include but are notlimited to: Reflective Waveguide Bragg Gratings (WBGs) such as FBGs(Fiber Bragg Gratings), and SFBGs (Sampled Fiber Bragg Gratings);Transmissive WBGs such as phase-shifted FBGs (with single or multiplepassbands); PZT-based FFP (fiber Fabry-Perot) filters; LCD-based FFP(fiber Fabry-Perot) filters; Microelectromechanical system (MEMS)-basedFP (Fabry-Perot) filters; Electro-optic FP filters (i.e., LCD-based,poled glass, poled crystals); Micro-ring resonators; Grating-assistedwaveguide couplers; Electro-optic waveguide coupler; MI (MichelsonInterferometer) waveguide-based filter; MZI (Mach-ZehnderInterferometer) waveguide-based filter; and Arrayed Waveguide Gratings(AWGs), etc.

The term “rapidly wavelength-tunable filter” for the purpose of thepresent invention refers to a device that provides the means for rapid(<50 μs) tuning of the optical filter passband across the spectral rangeof interest. The means include but are not limited to: thermal or straintuning, electro-optic tuning, and electro-absorptive modulation. Thedevice may have single or multiple wavelength passbands. The device maybe composed of a single optical filter or a set of such optical filters.Specific examples of rapidly wavelength-tunable filters include but arenot limited to: Reflective Waveguide Bragg Gratings (WBGs) such astunable FBGs (Fiber Bragg Gratings), and tunable SFBGs (Sampled FiberBragg Gratings); Transmissive WBGs such as phase-shifted tunable FBGs(with single or multiple passbands); PZT-based FFP (fiber Fabry-Perot)filters; LCD-based FFP (fiber Fabry-Perot) filters;Microelectromechanical system (MEMS)-based FP (Fabry-Perot) filters;Electro-optic FP filters (i.e. LCD-based, poled glass, poled crystals);Micro-ring resonators (i.e. poled glass, electro-absorptive material);Grating-assisted tunable waveguide couplers; Electro-optic waveguidecoupler; Tunable Michelson interferometer (MI) waveguide-based filter;Tunable MZI (Mach-Zehnder Interferometer) waveguide-based filter; andTunable AWGs, etc.

The term “optical feedback coupling device” for the purpose of thepresent invention refers to a device for combining the optical feedbackfrom transmissive and reflective filters in the same laser resonatorcavity. Specific examples of such devices include, but are not limitedto, N-port optical circulators (N=3, 4, etc.), fused fiber couplers, andwaveguide-Y junctions, etc.

The term “gain medium” for the purpose of the present invention refersto a medium that provides gain to the optical signal. Examples of suchmediums include, but are not limited to doped fiber optic waveguides orsemiconductor optical amplifiers.

The term “pump source” for the purposes of the present invention refersto an optical or electrical tunable or switchable emission.

The term “wavelength comb” for the purposes of the present inventionrefers to refers to a set of wavelengths that are equally spaced orwhose spacing are multiples of a common interval.

Description

Before the preferred embodiments are described, we will first discussthe invention in a general fashion. It should be appreciated that thereare two distinct embodiments of the invention. The first embodimentaddresses precisely wavelength switchable narrow linewidth lasers. Thisfirst embodiment is mainly used in WDM telecommunication systems. Thesecond embodiment addresses precisely wavelength tunable narrowlinewidth lasers. This second embodiment is mainly used for tuning “onand off” narrow absorption lines in spectroscopic measurements,including the monitoring of resonantly absorbing species in DIAL(differential absorption LTDAR)-type applications.

In the precisely wavelength switchable narrow linewidth laserembodiment, the design is based on two narrow band filters: (1) a stablemulti-wavelength filter where each selected wavelength is preciselymatched to a standard grid of WDM channels, and (2) a rapidly (<50 μs)wavelength-tunable filter. In the 1.5 um telecommunications spectralregion, both narrow linewidth filters should have bandwidths of at least5 GHz (preferably <1 GHz). These bandwidths are determined by theprecision required of each channel in the standard grid of WDM channels.This laser source can have single- or multiple-wavelength rapidlywavelength-switchable emission in the channels specified by themulti-wavelength grid filter.

In the precisely wavelength tunable narrow linewidth laser embodiment,the device is based on two filters: (1) a stable multi-wavelength filterwhere each selected wavelength is precisely matched to a pre-designatedset of wavelength channels (such as that which coincides withsignificant absorption peaks of chosen molecular species), and (2) arapidly wavelength-tunable filter. The multi-wavelength filter shouldhave individual wavelength channels that have bandwidths that are atleast as broad as the tuning range desired (i.e. bandwidth of molecularabsorption feature). The rapidly wavelength-tunable filter should have abandwidth that is at least 10 times (preferably<20 times) narrower thanthe bandwidth of the above-mentioned absorption feature. In thespectroscopy applications for the 1.5 um near-infrared spectral region,the significant molecular absorption features are normally 10-30 GHzwide, so the multi-wavelength filter wavelength channels should have abandwidth that is at least as wide (10-30 GHz), while the rapidlytunable wavelength filter should have a bandwidth less than 1 GHz(preferably<100 MHz). This laser source can have single- ormultiple-wavelength rapidly wavelength-tunable emission in thewavelength channels specified by the multi-wavelength grid filter.

In either embodiment, the laser resonator can be wholly or in part ofring, or linear geometry. A pump source provides energy for the gainmedium and a means for coupling this energy to the gain medium isprovided. The gain medium may be a doped fiber optic waveguide or asemiconductor optical amplifier. The optical field in the laserresonator has a roundtrip path that is facilitated in part or wholly byvarious media which include, but are not limited to: optical fiber,planar waveguides in bulk glass, planar waveguides in semiconductors, aswell as free space open cavity resonators in MEMS-based designs.

In a ring geometry, transmissive filters may be directly inserted in theroundtrip path. In the linear geometry, reflective filters can bedirectly inserted in both ends of the cavity roundtrip path. In a cavitywhich is part linear and part ring, the means to couple the feedbackfrom both filters is provided by the optical feedback coupling device.An example of such a device is a 3-port circulator which transforms areflective FBG into a transmissive filter.

The multi-wavelength filter is stabilized through an ultrastablecompensation means that is usually accomplished through a referencewavelength wavelength_(ref) which is locked to a frequency standard.This reference wavelength(s) wavelength_(ref) is a subset of thepre-designated multi-wavelength grid. The multi-wavelength filter may bea set of discrete filters or a single filter.

The tunable wavelength filter may be a set of discrete filters or asingle tunable filter. The means for rapid tuning of wavelength-tunablefilter is provided.

The source design of the invention as shown, for example, in FIG. 1,uses a dual filter approach comprising a filter composed of a set offiber Bragg gratings (FBGs) that generates a wavelength combcorresponding to the ITU grid, and a second tunable filter composed of afiber Fabry-Perot (FFP-TF) that selects the desired wavelength channel.A precise and stable single or multi-frequency grid is set up by filters(usually by reflective narrow linewidth fiber Bragg gratings) in thelaser cavity while a (usually electronically-) tunable high-finessefilter (which may be transmissive or reflective) is actuated to selectthe desired wavelength output. The reflective alters (usually fiberBragg gratings) feed back preselected wavelengths while the transmissivefilter (usually a tunable fiber Fabry-Perot or FBGs in transmissionmode) adds a second level of wavelength selectivity, such that tuning ofthe transmission peaks enables selection of the emission wavelength.

Optical isolators are used, where appropriate, to ensure unidirectionaloperation and, in the case of embodiments employing FFP transmissivefilters, to prevent feedback arising from wavelengths reflected off thenon-transmissive FFP bands. Optical circulators, in combination withreflective filters (usually fiber Bragg gratings), are employed inspecific embodiments to allow for efficient low-loss precise wavelengthfeedback from the pre-designated set (single or multiple) ofwavelengths. Polarization control is introduced at appropriate places inthe cavity to ensure optimal output powers and stability.

Now that the basic concepts of the invention have been described, wewill now describe specific structures that utilize the teachings of thetwo embodiments above. Turning now to FIG. 1, a Sagnac ring laser cavityconfiguration is illustrated. Pump source 100 generates light beam 102at a preselected wavelength λ₁, such as 980 nm, which travels throughgain medium 105. Gain medium 105 is preferably a doped fiber opticwaveguide, such as Er:Silica, or a pigtailed Semiconductor OpticalAmplifier etc. Gain medium 105 may be placed in either arm of the laserconfiguration or in the transmissive filter loop. The wave travelsthrough fused fiber coupler 110 and is then partially reflected by fiberBragg grating 115. In a preferred embodiment, the reflection would bebetween 10% and 100%. Coupler 110 may be a waveguide Y, pigtail,multiple branch elements, N-port circulators, etc. FBGs may be any typeof reflective filters such as discrete FBGs, sampled gratings, aninterferometric reflective filter, an arrayed waveguide grating withreflective elements on output ports, tunable Bragg gratings as disclosedin U.S. patent application Ser. No. 09/246,125 etc. The wave thentravels back through fused fiber coupler 110 and gain medium 105 and isfurther tuned by fiber Fabry-Perot tunable filter (FFP) 120. FFP 120 maybe any sort of multiple or single-passband (notch) transmissive filtersuch as an FFP, sampled FBG, FBG in transmission mode, tunable Bragggratings as disclosed in U.S. patent application Ser. No. 09/246,125,waveguide ring resonators, etc. The output of fiber Fabry-Perot filter120 is fed into an optional optical isolators 121 and fused fibercoupler 110 and passed through fiber Bragg grating 115 to create output125 having an output wavelength λ₂.

The transmissive filter of the present invention may be an FFP withcoated fiber mirrors, FFP with FBG mirrors, phase-shifted FBG-basedtransmission filters, waveguide ring resonator filters, MEMs-based FFPs,etc. Electronically tunable filters may be PZT-tuned FBGs, EO-tunableFBGs, PZT-tuned FFP, MEMs-based tunable FFP, etc. FBGs may be singlefrequency, multiple frequency (sampled FBG), etc.

Preferably, a 0.08 nm linewidth fiber Bragg grating (FBG), with areflectivity of 25%, centered at 1531.08 in a Sagnac loop geometry maybe used. The fiber Fabry-Perot (FFP) provides intracavity wavelengthtunability to the source. The filter has a 20 GHz free spectral range(FSR) and a 20 MHz linewidth (LW). The isolators prevent unwantedreflective feedback from the high reflectivity FFP mirror surfaces.

An instrument-limited linewidth of 0.05 nm and 15 GHz of maximum tuningcentered in the vicinity of the FBG peak are demonstrated. The observedoutput powers (2 mW max), however, show 10% fluctuations, and spectralinstability (estimated to be approximately 1 GHz).

FIG. 1B shows the tuning curve for a 1.5 micron travelling wave laserarranged according to FIG. 1A.

FIG. 2A shows a linear cavity standing wave laser arrangement. A pumpsource generates light beam 200 at a preselected wavelength λ₁, such as980 nm, which travels through gain medium 205. Gain medium 205 may bedoped fiber optic waveguide, such as Er:Silica, or a pigtailedSemiconductor Optical Amplifier, etc. Gain medium 205 may be placed ineither arm of the laser configuration or in the transmissive filterloop. The wave interacts with fiber Bragg grating 210 and is partiallyreflected. In a preferred embodiment, the reflection would be between10% and 100%. FBGs may be any type of reflective filters, for examplebut not limited to, discrete FBGs, sampled gratings, an interferometricreflective filter, an arrayed waveguide grating with reflective elementson output ports, or tunable Bragg gratings as disclosed in U.S. patentapplication Ser. No. 09/246,125. Light beam 200 travels through gainmedium 205 and coupler 215 and is further tuned by fiber Fabry-Perottunable filter 220. Coupler 215 may be a fused fiber coupler, waveguideY, pigtail, multiple branch elements, N-port circulators, etc. FFP 220may be any sort of multiple or single-passband (notch) transmissivefilter such as an FFP, sampled FBG, FBG in transmission mode, waveguidering resonators, etc. The output of fiber Fabry-Perot filter 220 is fedinto an optional optical isolators 221 and coupler 215 and either passedthrough fiber Bragg grating 210 to create an output wavelength λ₂ 230 orpassed through coupler 215 to BB mirror 225 and reflected. Thereflection of BB mirror 225 causes the laser to oscillate at the pointswhere the modes coincide. In a preferred embodiment, mirror 225 has thefollowing properties, reflective at the pump wavelength between 10% and100%. The effective linear cavity FSR (not shown) will be larger thanthe FSR of the laser of FIG. 1A due to this vernier effect.

The standing wave laser of FIG. 2A exhibits 19 GHz of maximum tuningbased on the FFP 220 selected and the input voltage applied thereto.Within the limits of the optical spectrum analyzer resolution, nospectral peak instabilities are discerned. Output power fluctuations areapproximately 1%, suggesting high levels of stability due to minimallongitudinal mode competition within the FFPs 20 MHz transmission LW dueto the large effective FSR.

FIG. 2B shows the tuning curve for a 1.5 micron standing wave laserarranged according to FIG. 2A.

FIG. 3 shows a linear cavity standing wave laser arrangement. A pumpsource generates input light beam 300 at a preselected wavelength λ₁,such as 980 nm, which travels through gain medium 305. Gain medium 305may be a doped fiber optic waveguide, such as Er:Silica, or a pigtailedSemiconductor Optical Amplifier, etc. Gain medium 305 may be operatedthrough current injection (e.g. semiconductor laser amplifier), orpumped by optical means from any side, singly or bidirectionally. Gainmedium 305 may be placed in output segment arm 335, transmissive filterloop 340, or in segment arm 345, singly or multiply. The wave interactswith fiber Bragg grating (FBG) 310 and is partially reflected. FBG 310may be a single element comb reflective filter, discrete set ofreflective filters, tunable Bragg gratings as disclosed in U.S. patentapplication Ser. No. 09/246,125 etc. FBGs may be any type of reflectivefilters, for example but not limited to, discrete FBGs, sampledgratings, an interferometric reflective filter, or an arrayed waveguidegrating with reflective elements on output ports, tunable Bragg gratingsas disclosed in U.S. patent application Ser. No. 09/246,125 etc. Lightbeam 300 travels through coupler 315 and is further tuned by fiberFabry-Perot tunable filter 320. Light beam 300 may contact optionalpolarization control element 350, which may be in output segment arm335, transmissive filter loop 340, or in segment arm 345, singly ormultiply. Coupler 315 may be a 3 dB coupler, a fused fiber coupler,waveguide Y, pigtail, multiple branch elements, N-port circulators, etc.FFP 320 may be any sort of multiple or single-passband (notch)transmissive filter such as an FFP, sampled FBG, FBG in transmissionmode, waveguide ring resonators, etc. The output of fiber Fabry-Perotfilter 320 is fed through optional optical isolators 355 and intocoupler 315 and either passed through fiber Bragg grating 310 to createan output wavelength λ₂ 330 or passed through coupler 315 to BB mirror325 and reflected. The reflection of BB mirror 325 causes the laser tooscillate at the points where the modes coincide. In a preferredembodiment, mirror 325 has the following properties, reflective at thepump wavelength between 10% and 100%. The effective linear cavity FSR(not shown) will be larger than the FSR of the laser of FIG. 1A due tothis vernier effect.

The switchable fiber lasers of the present invention may be designedusing linear, ring, multiple cavity configurations, etc. Theconfigurations may be a linear cavity standing wave laser, a Sagnac ring(based on fused fiber coupler) travelling wave laser, a ring cavity(based on optical circulators), etc. The configuration may alsoincorporate multiple cavities. A feature of each of these configurationsis the use of an FFP having a switching speed of less than 1milliseconds, or less than 100 microseconds in another preferredembodiment, and most preferably less than 10 microseconds.

Although only switching between two pre-designated wavelengths has beenshown for the embodiments described above, it should be appreciated thatswitching between multiple precise DWDM channels is within the scope ofthe present invention. This switching is achieved by (1) dynamicallyadjusting the input voltage to the FFP; (2) dynamically adjusting theresonant frequency of the FBG; or (3) a combination of methods (1) and(2).

FIG. 4 shows a preferred embodiment of a standing wave laser of theinvention using pumped input light beam 400 at a preselected wavelengthλ₁, such as 980 nm, to produce output wavelength λ₂ 440. Gain medium 405may be a doped fiber optic waveguide, such as Er:Silica, or asemiconductor optical amplifier, etc. Gain medium 405 may be operatedthrough current injection (e.g. semiconductor laser amplifier), orpumped by optical means from any side, singly or bidirectionally. Gainmedium 405 may be placed in output segment arm 425 or in transmissivefilter loop 430, singly or multiply. FBGs 410 feed back pre-selectedwavelengths λ₁, and λ₂, while tunable fiber Fabry-Perot filter 420 addsa second level of wavelength selectivity, such that voltage tuning ofthe FFP transmission peaks, via an input voltage, enables selection ofthe desired single-wavelength output. In a preferred embodiment, the FBGreflectivity would be between 10% and 100%. FFP 420 may be any sort ofmultiple or single-passband (notch) transmissive filter such as an FFP,sampled FBG, FBG in transmission mode, tunable fiber Bragg gratings asdisclosed in U.S. patent application Ser. No. 09/246,125, waveguide ringresonators, etc. Light beam 400 also interacts with optionalpolarization control element 450, which may be in output segment arm 425or in transmissive filter loop 430, singly or multiply. The output ofFFP 420 is fed through optional optical isolators 435. FBGs 410 may be asingle element comb reflective filter, a discrete set of reflectivefilters, or a set of tunable fiber Bragg gratings as disclosed in U.S.patent application Ser. No. 09/246,125, depending on the required levelsof selectivity. Coupler 415 may be a 3 dB coupler, a fused fibercoupler, waveguide Y. pigtail, multiple branch elements, N-portcirculators, etc.

FIG. 5 shows an alternate embodiment of the invention using pumped inputbeam 500 at a preselected wavelength λ_(a), such as 980 nm, to produceoutput wavelength λ_(b) 540, such as 1.5 μm based on a 980 nm input.Input beam 500 engages WDM coupler 505 and travels through gain medium510, which may be a doped fiber optic waveguide, such as Er:Silica, or asemiconductor optical amplifier, etc. Gain medium 510 may be operatedthrough current injection (e.g. semiconductor laser amplifier), orpumped by optical means from any side, singly or bidirectionally. Gainmedium 510 may be placed in either arm of the laser configuration or inthe transmissive filter loop. FBGs 515 reflect pre-selected wavelengthsλ₁ and λ₂, while tunable fiber Fabry-Perot filter 530 adds a secondlevel of wavelength selectivity, such that voltage tuning of the FFPtransmission peaks enables selection of the desired single-wavelengthoutput. In a preferred embodiment, the FBG reflectivity would be between10% and 100%. FFP 530 may be any sort of multiple or single-passband(notch) transmissive filter such as an FFP, sampled FBG, FBG intransmission mode, tunable fiber Bragg gratings as disclosed in U.S.patent application Ser. No. 09/246,125, waveguide ring resonators, etc.The output of FFP 530 is fed through optional optical isolators 545.FBGs 515 may be a single element comb reflective filter, a discrete setof reflective filters, tunable Bragg gratings as disclosed in U.S.patent application Ser. No. 09/246,125, etc. depending on the requiredlevels of selectivity. Coupler 520 may be a 3 dB coupler, a fused fibercoupler, waveguide Y, pigtail, multiple branch elements, N-portcirculators, etc.

FIG. 6 shows a quasi-unidirectional travelling wave laser of the presentinvention using pump input light beam 600 at a preselected wavelengthλ_(a), such as 980 nm, to produce output wavelength λ_(b) 650. Gainmedium 605 may be doped fiber, such as Er:Silica, or a semiconductoroptical amplifier, etc. Gain medium 605 may be operated through currentinjection (e.g. semiconductor laser amplifier), or pumped by opticalmeans from any side, singly or bidirectionally. Gain medium 605 may beplaced in first segment arm 635, in transmissive filter loop 640, or insecond segment arm 645, singly or multiply. FFP 620 may be any sort ofmultiple or single-passband (notch) transmissive filter such as an FFP,sampled FBG, FBG in transmission mode, waveguide ring resonators, etc.Light beam 600 also interacts with optional optical isolators 630 intransmissive filter loop 640. Optional polarization control element 625may be placed in first segment arm 635, transmissive filter loop 640, orin second segment arm 645, singly or multiply. FBGs 615 may be a singleelement comb reflective filter, a discrete set of reflective filters,tunable Bragg gratings as disclosed in U.S. patent application Ser. No.09/246,125, etc. depending on the required levels of selectivity.Coupler 610 may be performed by a 3 dB coupler, a fused fiber coupler,waveguide Y, pigtail, multiple branch elements, N-port circulators, etc.

FIG. 7 shows a quasi-unidirectional travelling wave laser of the presentinvention using pump input light beam 700 at a preselected wavelengthλ_(a), such as 980 nm, to produce output wavelength λ_(b) 750. Gainmedium 705 may be a doped fiber optic waveguide, such as Er:Silica, or asemiconductor optical amplifier, etc. Gain medium 705 may be operatedthrough current injection (e.g. semiconductor laser amplifier), orpumped by optical means From any side, singly or bidirectionally. Gainmedium 705 may be placed in segment arm 735 or in transmissive fillerloop 740, singly or multiply. FFP 720 may be any sort of multiple orsingle-passband (notch) transmissive filter such as an FFP, sampled FBG,FBG in transmission mode, tunable Bragg gratings as disclosed in U.S.patent application Ser. No. 09/246,125, waveguide ring resonators, etc.Light beam 700 also interacts with optional optical isolators 730 intransmissive filter loop 740. Optional polarization control element 725may be placed in segment arm 735 or in transmissive filter loop 740,singly or multiply. FBGs 715 may be a single element comb reflectivefilter, a discrete set of reflective filters, tunable Bragg gratings asdisclosed in U.S. patent application Ser. No. 09/246,125, etc. dependingon the required levels of selectivity. Coupler 710 may be performed by a3 dB coupler, a fused fiber coupler, waveguide Y, pigtail, multiplebranch elements, N-port circulators, etc.

FIG. 8 shows a wavelength-switchable and tunable fiber laser of thepresent invention using a three-port circulator and pump input lightbeam 800 at a preselected wavelength λ_(a) such as 980 nm, to produceoutput wavelength λ_(b) 830. The three ports are labeled P₁, P₂ and P₃.Gain medium 805 may be a doped fiber optic waveguide, such as Er:Silica,or a semiconductor optical amplifier, etc. Gain medium 805 may beoperated through current injection (e.g. semiconductor laser amplifier),or pumped by optical means from any side, singly or bidirectionally.Light beam 800 passes through optional optical isolators 815, coupler810 and FFP 820. FFP 820 may be any sort of multiple or single-passband(notch) transmissive filter such as an FFP, sampled FBG, FBG intransmission mode, tunable Bragg gratings as disclosed in U.S. patentapplication Ser. No. 09/246,125, waveguide ring resonators, etc. FBGs825 may be a single element comb reflective filter, a discrete set ofreflective filters, tunable Bragg gratings as disclosed in U.S. patentapplication Ser. No. 09/246,125, etc. depending on the required levelsof selectivity. Coupler 810 may be a 3 dB coupler, a fused fibercoupler, waveguide Y, pigtail, multiple branch elements, N-portcirculators, etc. Optional polarization control elements (not pictured)may be added to the loop singly or multiply.

FIG. 9 shows a wavelength-switchable and tunable fiber laser of thepresent invention using a four-port circulator with input 900 at apreselected wavelength λ_(a) such as 980 nm to produce output wavelengthλ_(b) 930. The four ports are indicated as P₁, P₂ P₃ and P₄. Gain medium905 may be a doped fiber optic waveguide, such as Er:Silica, or asemiconductor optical amplifier, etc. Gain medium 905 may be operatedthrough current injection (e.g. semiconductor laser amplifier), orpumped by optical means from any side, singly or bidirectionally. Thelight beam passes through optional optical isolators 915, coupler 910and FFP 920. FFP 920 may be any sort of multiple or single-passband(notch) transmissive filter such as an FFP, sampled FBG, FBG intransmission mode, tunable Bragg gratings as disclosed in U.S. patentapplication Ser. No. 09/246,125, waveguide ring resonators, etc. FBGs925 may be a single element comb reflective filter, a discrete set ofreflective filters, tunable Bragg gratings as disclosed in U.S. patentapplication Ser. No. 09/246,125, etc. depending on the required levelsof selectivity. Coupler 910 may be a 3 dB coupler, a fused fibercoupler, waveguide Y, pigtail, multiple branch elements, N-portcirculators, etc. Optional polarization control elements (not pictured)may be added to the loop singly or multiply. The reflection of mirror935 causes the laser to oscillate at the points where the modescoincide.

FIG. 10 shows a wavelength-switchable and tunable fiber laser of thepresent invention using a four-port circulator or two cascaded 3-portswith input 1000 at a preselected wavelength λ_(a), such as 980 nm, toproduce output λ_(b) 1030. The four ports are indicated as P₁, P₂ P₃ andP₄. Gain medium 1005 may be a doped fiber optic waveguide, such asEr:Silica, or a semiconductor optical amplifier, etc. Gain medium 1005may be operated through current injection (e.g. semiconductor laseramplifier), or pumped by optical means from any side, singly orbidirectionally. The light beam passes through optional opticalisolators 1015, coupler 1010 and FFPs 1020. FBGs 1025 may be a singleelement fixed-frequency-comb reflective filter, a discrete set ofreflective fixed-frequency filters, tunable Bragg gratings as disclosedin U.S. patent application Ser. No. 09/246,125, etc. depending on therequired levels of selectivity. FFPs 1020 may be a single elementtunable frequency-comb filter or a discrete set of reflective tunablefilters. FFPs 1020 may be any sort of multiple or single-passband(notch) transmissive filter such as an FFP, sampled FBG, FBG intransmission mode, tunable Bragg gratings as disclosed in U.S. patentapplication Ser. No. 09/246,125, waveguide ring resonators, etc. Coupler1010 may be a 3 dB coupler, a fused fiber coupler, waveguide Y, pigtail,multiple branch elements, N-port circulators, etc. Optional polarizationcontrol elements (not pictured) may be added to the loop singly ormultiply.

FIG. 11 shows a wavelength-switchable and tunable fiber laser of thepresent invention using a four-port circulator and pump input light beam1100 at a preselected wavelength λ_(a), such as 980 nm, to produceoutput wavelength λ_(b) 1130. The four ports are indicated as P₁, P₂ P₃and P₄. Gain medium 1105 may be a doped fiber optic waveguide, such asEr:Silica, or a semiconductor optical amplifier, etc. Gain medium 1105may be operated through current injection (e.g. semiconductor laseramplifier), or pumped by optical means from any side, singly orbidirectionally. The light beam passes through optional opticalisolators 1115. FBGs 1125 may be a single element fixed-frequency-combreflective filter, a discrete set of reflective fixed-frequency filters,tunable Bragg gratings as disclosed in U.S. patent application Ser. No.09/246,125, etc. depending on the required levels of selectivity. FFPs1120 may be a single element tunable frequency-comb filter or a discreteset of reflective tunable filters. FFPs 1120 may be any sort of multipleor single-passband (notch) transmissive filter such as an FFP, sampledFBG, FBG in transmission mode, tunable Bragg gratings as disclosed inU.S. patent application Ser. No. 09/246,125, waveguide ring resonators,etc. FBGs 1125 and FFPs 1120 are joined via coupler 1135. Coupler 1135and second coupler 1110 may each be a 3 dB coupler, a fused fibercoupler, waveguide Y, pigtail, multiple branch elements, N-portcirculators, etc. Optional polarization control elements (not pictured)may be added to the loop singly or multiply.

FIG. 16 illustrates a compact hybrid rapidly wavelength-switchable andwavelength-tunable continuous wave (CW) or pulsed source constructed inaccordance with yet another embodiment of the invention. In thisembodiment, the components are fabricated from various technologies,namely: semiconductor, micro-optic, fiber-optic and glass waveguidetechnologies. This embodiment is a compact planar lightwave circuitdesign fabricated on a semiconductor (such as InP or Silicon) or glasssubstrate. Special anisotropic etching techniques (such as DeepRIE—Reactive Ion Etching) capable of etching deep features (>100 μm) areutilized in this embodiment. These anisotropic etching techniques enablehybrid integration of components through high-tolerance self-alignmenttechniques. As may be seen, optical pump source 1600 (if necessary) isprovided for optical pumping of doped glass gain medium 1605. Anoptional device for coupling in optical pump source 1610 is provided forcoupling optional pump source 1600 to doped glass gain medium 1605. Anoutput signal 1630 is generated by the device. In a preferredembodiment, gain medium 1605 is a rare-earth doped glass, including butnot limited to Er:Silica, Er/Yb:Silica, Yb:Silica, Er:ZBLAN,Er/Pr:ZBLAN, Er/Yb ZBLAN) or a semiconductor optical amplifier. Awavelength-tunable or wavelength-stabilized Bragg grating 1625(single-wavelength Bragg grating or multiple-wavelength sampled Bragggrating) is disposed between source 1600 and gain medium 1605. A rapidlywavelength-tunable filter 1620 is disposed in the transmissive loopwhich is connected to gain medium 1605 via a waveguide Y-branch oroptical field coupling device 1635. An optional amplitude or phasemodulation device 1299 may be disposed between gain medium 1605 andwavelength-tunable or wavelength-stabilized Bragg grating 1625. Optionaloptical isolators 1650 are disposed on either side of rapidlywavelength-tunable filter 1220. A coupling device 1680 is provided forfacilitating optical fiber to waveguide coupling. One example of acoupling device is V-grooves or any other coupling device known in theart.

FIG. 17 illustrates a compact hybrid rapidly wavelength-switchable andwavelength-tunable continuous wave (CW) or pulsed source constructed inaccordance with yet another embodiment of the invention. As may be seen,components fabricated from various technologies, namely: semiconductor,micro-optic, fiber-optic and glass waveguide technologies are utilizedin this embodiment. A compact planar lightwave circuit design isfabricated on a semiconductor (such as InP or Silicon) or glasssubstrate. Special anisotropic etching techniques such as Deep RIE andReactive Ion Etching are utilized. These techniques are capable ofetching deep features (>100 μm) which enable hybrid integration ofcomponents through high-tolerance self-alignment techniques. As may beseen, optical pump source 1700 (if necessary) is provided for opticalpumping of doped glass gain medium 1705. An optional device for couplingin optical pump source 1710 is provided for coupling optional pumpsource 1700 to doped glass gain medium 1705. An output signal 1730 isgenerated by the device. In a preferred embodiment, gain medium 1705 isa rare-earth doped glass, including but not limited to Er:Silica,Er/Yb:Silica, Yb:Silica, Er:ZBLAN, Er/Pr:ZBLAN, Er/Yb ZBLAN) or asemiconductor optical amplifier. A wavelength-tunable orwavelength-stabilized Bragg grating 1725 (single-wavelength Bragggrating or multiple-wavelength sampled Bragg grating) is disposedbetween source 1700 and gain medium 1705. A tunable array waveguidegrating (AWG) 1720 is disposed in the transmissive loop which isconnected to gain medium 1705 via a optical field coupling device 1735.An optional amplitude or phase modulation device 1799 may be disposedbetween gain medium 1705 and tunable array waveguide grating (AWG) 1720.A coupling device 1780 is provided for facilitating optical fiber towaveguide coupling. One example of a coupling device is V-grooves or anyother coupling device known in the art. As may be seen, aphase-modulated region 1790 is provided in tunable array waveguidegrating (AWG) 1720. This phase-modulated region 1790 may include, but isnot limited to: electro-absorptive material, PZT strained regions, andelectro-optic material.

Turning now to FIG. 18, an alternate embodiment is illustrated. As maybe seen, optical pump source 1800 (if necessary) is provided for opticalpumping of doped glass gain medium 1805. An optional device for couplingin optical pump source 1815 is provided for coupling optional pumpsource 1800 to doped glass gain medium 1815. In a preferred embodiment,gain medium 1815 is a rare-earth doped glass, including but not limitedto Er:Silica, Er/Yb:Silica, Yb:Silica, ErZBLAN, Er/Pr:ZBLAN, Er/YbZBLAN) or a semiconductor optical amplifier. A wavelength-tunable orwavelength-stabilized multiwavelength grid filter 1810(single-wavelength Bragg grating or multiple-wavelength sampled Bragggrating) is disposed distal from source 1800 and gain medium 1815. Arapidly wavelength tunable filter 1820 is disposed in the laser cavityresonator.

Turning now to FIG. 19, an alternate embodiment is illustrated. As maybe seen, optical pump source 1900 (if necessary) is provided for opticalpumping of doped glass gain medium 1905. An optional device for couplingin optical pump source 1915 is provided for coupling optional pumpsource 1900 to doped glass gain medium 1915. In a preferred embodiment,gain medium 1915 is a rare-earth doped glass, including but not limitedto Er:Silica, Er/Yb:Silica, Yb:Silica, ErZBLAN, Er/Pr:ZBLAN, Er/YbZBLAN) or a semiconductor optical amplifier. A wavelength-tunable orwavelength-stabilized multiwavelength grid filter 1910(single-wavelength Bragg grating or multiple-wavelength sampled Bragggrating) is disposed distal from source 1900 and gain medium 1915. Arapidly wavelength tunable filter 1920 is disposed in the laser cavityresonator.

As shown in FIG. 12, when the FFP free spectral range (FSR)<Δf, anecessary condition for successful operation of the fiber laser atprecisely λ-switchable narrow linewidth outputs is given by: Min(|n Δf-mFSR|)>δ_(tolerance), where n and m are integers,δf_(tolerance)˜½(δf_(FFP)+δf_(FBG)) and δf_(FFP) and δf_(FBG).

One example of a design wherein the FFP FSR<Δf involves eight channelswith Δf=50 GHz. A coordinate FFP has an FSR of about 44 GHz, whichrequires a δf_(tolerance) of about 5 GHz.

Multi-wavelength switchable operation has also been illustrated in FIG.13 for cases where FSR>Δf. To obtain the diagram of FIG. 13, forexample, two FBGs may be used with peak reflectivities of approximately30 dB and 3 dB linewidths of approximately 0.4 nm (50 GHz) at centerwavelengths of 1551.68 nm (λ₁) and 1554.10 nm (λ₂). The FFP filter has a5 GHz linewidth and a 5 THz (40 nm) free spectral range corresponding toa finesse of ˜1000. Isolators are placed inside the loop to ensureunidirectional operation and to prevent feedback from wavelengthsreflected off the bandstop regions of the FFP transmission spectrum. Theoutput spectra may be easured using an Ando AQ-6315A optical spectrumanalyzer which has a resolution of 0.05 nm, or by using a comparabledevice.

The speed of switching may be measured using a tunable bandpass filterand an isolator in front of a 1 GHz InGaAs photodetector. The bandpassfilter may be tuned to one channel or another, to give a 30 dB rejectionbetween the two channels. Rapid switching between the two wavelengths isaccomplished with the use of a 1 KHz square wave input voltage source(˜60 ns rise time) to drive the tunable FFP.

FIGS. 14A and 14B show the results of a two-channel switchingdemonstration and a spectrum range from 1524.50 nm to 1559.50 nm. FIG.14A shows a wavelength peak at approximately 1531.1 nm with −0.4 dB,approximately 0.08 nm linewidth, and a range of 1.2 dB. The voltage wasset in FIG. 14A at approximately 22.1 V+/−0.7 V. The resultant SMSR was24 dB. FIG. 14B shows a wavelength peak at approximately 1552.6 nm with−3 dB, approximately 0.35 nm linewidth, and a range of 12 dB. Thevoltage was set in FIG. 14B at approximately 22.1 V+/−0.7 V. Theresultant SMSR was 16 dB. The difference in SMSR results from asymmetryin Bragg grating strengths. The FFP for both FIGS. 14A and 14B had afinesse of approximately 1000 and a FSR of approximately 20 GHz.

FIGS. 15A and 15B depict the spectral data for two different voltagesettings applied to a PZT element of the FFP with a spectrum range from1550.50 nm to 1555.50 nm. Clear switching between the wavelengthscorresponding to the peaks of the FBGs is observed. FIG. 15A shows thata 1551.7 nm (λ₁) signal is selected for V_(FFP)=33.6 V+/−0.01 V. FIG.15B shows that the emission wavelength switched to 1554.1 nm (λ₂) whenthe FFP voltage 34.1 V+/−0.01 V. About 20 GHz (0.16 nm) of fine tuning,which corresponds to approximately 30 mV changes in the tunable filtervoltage (V_(FFP)), is observed around the center wavelengths λ₁,(1551.76 nm) and λ₂ (1554.17 nm). Reflective peaks of both FBGs arecentered on the ITU grid with a range of 30 dB (>99%), −25 dB, and alinewidth of 0.7 nm. For both wavelengths, output power fluctuations areless than 1% whenever one of the multiple order FFP transmission peaksis coincident with either one of the two grating reflective bands. Bothchannels also have side mode suppression ratios of approximately 40 dB.The FFP for both FIGS. 15A and 15B had a finesse of approximately 1000and a FSR of approximately 5 THz.

When the FFP transmission peak is not tuned to either grating'sreflective bands, i.e., FFP-unassisted emission at 1551 nm (λ₁), largeoutput power fluctuations of approximately 50% is exhibited. This effectis caused by a combination of several factors: slight differences inreflectivity between the two gratings, the formation of an effectivelaser cavity defined by the gratings on one end and stray reflectionsfrom the 3 dB coupler on the other, and to the gain coefficientdifferential between λ₁, and λ₂, which is estimated to be as much asapproximately 1 m⁻¹ for similar Er:Silica fibers pumped to highinversion levels.

Instrument limited linewidths of 0.05 nm are also observed. Linewidthmeasurements using scanning Fabry-Perot interferometry and self-homodynetechniques for similarly-configured fiber laser sources as shown in FIG.5 are expected to yield instantaneous linewidths on the order of 1 KHz.

The switching time between λ₁, and λ₂ was measured to be approximately25 μs based on a capacitive loading at the FFP PZT inputs. The switchingtime between the two wavelengths (separated by 300 GHz) is moreaccurately 18 μs, and may reach sub-μs speeds with the use ofBessel-function pre-filtered drivers for the FFP.

In an alternate embodiment of the present invention, the desired comb ofwavelength channels may be obtained from a single sampled FBG. Inaddition, the accuracy of the switched wavelengths may be assured byusing tighter (i.e. <10 GHz) 0.5 dB bandwidth specifications for theFBGs. Highly stabilized fixed FFPs may also be used to generate the ITUgrid directly.

Additionally, the laser cavity round trip time may be further reduced byreducing laser cavity lengths to approximately 1 m by using high dopingdensity Er/Yb:Silica fibers or semiconductor optical amplifiers. For a16-wavelength (50 GHz channel separation) switchable source using an FFPwith an optimized FSR switching speed of 5 μs, the switching time isapproximately 0.75 μs.

While the above discussion has focused on the switching between twowavelengths λ₁ and λ₂ or λ_(a) and λ_(b), it should be appreciated thatrunability from λ₁ to λ_(n) is achievable by utilizing an AC inputvoltage having the correct frequency and magnitude.

It should be appreciated that numerous embodiments describe a fiberlaser using only one fiber Fabry-Perot filter (FFP) in combination withdiscrete fiber Bragg gratings (FBGs). Among the advantages of usingFBGs, we can list the following:

-   -   (a) Cost of FBGs: Readily available narrow linewidth FBGs are at        least ten times cheaper than FFPs. In combination with        high-finesse FFPs, our laser source offers price/performance        advantages over prior art devices. In addition, the use of one        sampled FBG instead of multiple discrete FBGs provides for the        entire wavelength comb in an inexpensive manner.    -   (b) Wavelength stability of FBGs: Prior art devices make use of        one wavelength-tunable FFP and a fixed FFP (for wavelength        tunable devices), this combination makes it difficult to prevent        wavelength drift from the FFPs PZT tuning assembly, consequently        requiring the use of stable external “wavelength lockers.” In        contrast, our source inherently provides such stable wavelength        filters intra-cavity through the use of fixed-wavelength fiber        Bragg gratings.

Although the present invention has been fully described in conjunctionwith the preferred embodiment thereof with reference to the accompanyingdrawings, it is to be understood that various changes and modificationsmay be apparent to those skilled in the art. Such changes andmodifications are to be understood as included within the scope of thepresent invention as defined by the appended claims, unless they departtherefrom.

1. A precisely and rapidly wavelength-tunable CWR continuous wave (CW)or pulsed laser, said laser comprising: a gain medium; a pump source,said pump source for providing light to said gain medium; a laser cavityresonator having a transmissive filter loop and at least a first arm,said gain medium disposed in said laser cavity resonator and said pumpsource optically aligned with said laser cavity resonator; a rapidlywavelength-tunable filter disposed in said laser cavity resonatortransmissive filter loop, wherein said wavelength-tunable filter istunable by application of voltage across said wavelength-tunable filter;a multi-wavelength grid filter disposed in said laser cavity resonatorfirst arm; and means for removing an output signal from said lasercavity resonator.
 2. The precisely and rapidly wavelength-tunable CWRpulsed laser recited in claim 1, wherein said laser cavity resonator hasa transmissive filter loop and at least a first arm; said rapidlywavelength-tunable filter disposed in said transmissive filter loop; andsaid multi-wavelength grid filter disposed in said first arm.
 3. Theprecisely and rapidly wavelength-tunable CWR continuous wave (CW) orpulsed laser recited in claim 2 1, further comprising a second arm insaid laser cavity resonator, and second arm coupled to said first armand said transmissive filter loop.
 4. The precisely and rapidlywavelength-tunable CWR continuous wave (CW) or pulsed laser recited inclaim 3, further comprising a mirror disposed at an end of said secondarm distal from said transmissive filter loop.
 5. The preciselywavelength-tunable laser recited in claim 2 1, further comprising anoptical coupling feedback device for combining optical feedback fromsaid transmissive filter loop and said at least a first arm.
 6. Theprecisely wavelength-tunable laser recited in claim 5, wherein saidoptical feedback coupling device is selected from the group consistingof: N-port optical circulators, fused fiber couplers, and waveguide-Yjunctions.
 7. The precisely wavelength-tunable laser recited in claim 5,further comprising a second arm in said laser cavity resonator, saidsecond arm coupled to said first arm and said transmissive filter loopby said optical coupling feedback device.
 8. The precisely and rapidlywavelength-tunable CWR continuous wave (CW) or pulsed laser recited inclaim 1, wherein said multiwavelength grid filter is selected from thegroup consisting of Reflective Waveguide Bragg Gratings (WBGs); FBGs(Fiber Bragg Gratings); SWBG (Sampled Waveguide Bragg Gratings); SFBGs(Sampled Fiber Bragg Gratings); Transmissive WBGs; phase-shifted FBGs(with single or multiple passbands); Fabry-Perot (FP) micro-etalons andfilters; fiber FP filters and micro-optic; and micro-machined FPfilters; semiconductor FP filters; Micro-ring resonators; waveguidecouplers; Interferometric-waveguide-based filters; MichelsonInterferometric (MI) waveguide filters; Mach-Zehnder Interferometric(MZI) waveguide based filters; Arrayed Waveguide Gratings (AWGs); andpolarization interferometric (PI) waveguide-based filters; fiber loopmirrors; and bulk-optic -based PI filters.
 9. The precisely and rapidlywavelength-tunable CWR continuous wave (CW) or pulsed laser recited inclaim 1, wherein said rapidly wavelength-tunable filter is selected fromthe group consisting of tunable Reflective Waveguide Bragg Gratings(WBGs); tunable FBGs (Fiber Bragg Gratings); tunable SWBG (SampledWaveguide Bragg Gratings); tunable SFBGs (Sampled Fiber Bragg Gratings);tunable Transmissive WBGs; phase-shifted tunable FBGs (with single ormultiple passbands); tunable Fabry-Perot (FP) micro-etalons and filters;PZT-based fiber FP (FFP) filters; Liquid Crystal (LC)-based fiber FP(FFP) filters; Microelectromechanical Systems (MEMS)-based FP filters;electro-optic FP filters; current injection or optically tunedsemiconductor FP filter; tunable Micro-ring resonators; tunablewaveguide couplers; grating-assisted waveguide couplers andelectro-optic waveguide couplers; tunableInterferometric-waveguide-based filters; tunable MichelsonInterferometric (MI) waveguide filters; tunable Mach-ZehnderInterferometric (MZI) waveguide based filters; tunable Arrayed WaveguideGratings (AWGs); and tunable polarization interferometric (PI)waveguide-based filters; fiber loop mirrors with tunable electro-optic(EO) birefringent elements; fiber loop mirrors with tunablesemiconductor birefringent elements; and tunable bulk-optic -based PIfilters.
 10. The precisely and rapidly wavelength-tunable CWR continuouswave (CW) or pulsed laser recited in claim 1, wherein said gain mediumis selected from the group consisting of: doped fiber optic waveguidesand semiconductor optical amplifiers.
 11. The precisely and rapidlywavelength-tunable CWR continuous wave (CW) or pulsed laser recited inclaim 1, further comprising means for tuning said rapidlywavelength-tunable filter.
 12. The precisely wavelength-tunable laserrecited in claim 11, wherein said means for tuning said rapidlywavelength-tunable filter is selected from the group consisting of:electro-optic tuning devices, strain-tuned tuning devices andelectro-absorptive tuning devices.
 13. The precisely and rapidlywavelength-tunable CWR continuous wave (CW) or pulsed laser recited inclaim 1, wherein said rapidly wavelength-tunable filter has a tuningspeed of less than 50 μs.
 14. The precisely and rapidlywavelength-tunable CWR continuous wave (CW) or pulsed laser recited inclaim 1 13, further comprising means for tuning said rapidlywavelength-tunable filter.
 15. A precisely wavelength-tunable laser,said laser comprising: a gain medium; a pump source, said pump sourcefor providing light to said gain medium; a laser cavity resonator havinga transmissive filter loop and at least a first arm, said gain mediumdisposed in said laser cavity resonator and said pump source opticallyaligned with said laser cavity resonator; a rapidly wavelength-tunablefilter disposed in said laser cavity resonator transmissive filter loop,wherein said wavelength-tunable filter is tunable by application ofvoltage across said wavelength-tunable filter; a multi-wavelength gridfilter disposed in said laser cavity resonator first arm, wherein saidmulti-wavelength grid filter is selected from the group consisting ofReflective Waveguide Bragg Gratings (WBGs); FBGs (Fiber Bragg Gratings);SWBG (Sampled Waveguide Bragg Gratings); SFBGs (Sampled Fiber BraggGratings); Transmissive WBGs; phase-shifted FBGs (with single ormultiple passbands); Fabry-Perot (FP) micro-etalons and filters; fiberFP filters and micro-optic; and micro-machined FP filters; semiconductorFP filters; Micro-ring resonators; waveguide couplers;Interferometric-waveguide-based filters; Michelson Interferometric (MI)waveguide filters; Mach-Zehnder Interferometric (MZI) waveguide basedfilters; Arrayed Waveguide Gratings (AWGs); and polarizationinterferometric (PI) waveguide-based filters; fiber loop mirrors; andbulk-optic-based PI filters; and means for removing an output signalfrom said laser cavity resonator.
 16. The precisely wavelength-tunablelaser recited in claim 15, wherein said laser cavity resonator has atransmissive filter loop and at least a first arm; said rapidlywavelength-tunable filter disposed in said transmissive filter loop; andsaid multi-wavelength grid filter disposed in said first arm.
 17. Theprecisely wavelength-tunable laser recited in claim 16 15, furthercomprising a second arm in said laser cavity resonator, and second armcoupled to said first arm and said transmissive filter loop.
 18. Theprecisely wavelength-tunable laser recited in claim 17, furthercomprising a mirror disposed at an end of said second arm distal fromsaid transmissive filter loop.
 19. The precisely wavelength-tunablelaser recited in claim 16 15, further comprising an optical couplingfeedback device for combining optical feedback from said transmissivefilter loop and said at least a first arm.
 20. The preciselywavelength-tunable laser recited in claim 19, wherein said opticalcoupling feedback device is selected from the group consisting of:N-port optical circulators, fiber fused couplers, and waveguide-Yjunctions.
 21. The precisely wavelength-tunable laser recited in claim19, further comprising a second arm in said laser cavity resonator, saidsecond arm coupled to said first arm and said transmissive filter loopby said optical coupling feedback device.
 22. The preciselywavelength-tunable laser recited in claim 15, wherein said rapidlywavelength-tunable filter is selected from the group consisting oftunable Reflective Waveguide Bragg Gratings (WBGs); tunable FBGs (FiberBragg Gratings); tunable SWBG (Sampled Waveguide Bragg Gratings);tunable SFBGs (Sampled Fiber Bragg Gratings); tunable Transmissive WBGs;phase-shifted tunable FBGs (with single or multiple passbands); tunableFabry-Perot (FP) micro-etalons and filters; PZT-based fiber FP (FFP)filters; Liquid Crystal (LC)-based fiber FP (FFP) filters;Microelectromechanical Systems (MEMS)-based FP filters; electro-optic FPfilters; current injection or optically tuned semiconductor FP filter,tunable Micro-ring resonators; tunable waveguide couplers;grating-assisted waveguide couplers and electro-optic waveguidecouplers; tunable Interferometric-waveguide-based filters; tunableMichelson Interferometric (MI) waveguide filters; tunable Mach-ZehnderInterferometric (MZI) waveguide based filters; tunable Arrayed WaveguideGratings (AWGs); and tunable polarization interferometric (PI)waveguide-based filters; fiber loop mirrors with tunable electro-optic(EO) birefringent elements; fiber loop mirrors with tunablesemiconductor birefringent elements; and tunable bulk-optic-based PIfilters.
 23. The precisely wavelength-tunable laser recited in claim 15,wherein said gain medium is selected from the group consisting of: dopedfiber optic waveguides and semiconductor optical amplifiers.
 24. Theprecisely wavelength-tunable laser recited in claim 15, furthercomprising means for tuning said rapidly wavelength-tunable filter. 25.The precisely wavelength-tunable laser recited in claim 24, wherein saidmeans for tuning said rapidly wavelength-tunable filter is selected fromthe group consisting of: electro-optic tuning devices, strain-tunedtuning devices and electro-absorptive tuning devices.
 26. The preciselywavelength-tunable narrow linewidth laser recited in claim 15, whereinsaid rapidly wavelength-tunable filter has a tuning speed of less than50 μs.
 27. A precisely wavelength-tunable laser, said laser comprising:a gain medium; a pump source, said pump source for providing light tosaid gain medium; a laser cavity resonator, said gain medium disposed insaid laser cavity resonator and said pump source optically aligned withsaid laser cavity resonator; a rapidly wavelength-tunable filterdisposed in said laser cavity resonator, wherein said wavelength-tunablefilter is tunable by application of voltage across saidwavelength-tunable filter; a multi-wavelength grid filter disposed insaid laser cavity resonator; and means for removing an output signalfrom said laser cavity resonator, wherein said laser cavity resonatorhas a gain loop and at least a first arm and a second arm attached tosaid gain loop; said rapidly wavelength-tunable filter disposed in saidfirst arm; and said multi-wavelength grid filter disposed in said secondarm.
 28. The precisely wavelength-tunable laser recited in claim 27,wherein said multiwavelength grid filter is selected from the groupconsisting of Reflective Waveguide Bragg Gratings (WBGs); FBGs (FiberBragg Gratings); SWBG (Sampled Waveguide Bragg Gratings); SFBGs (SampledFiber Bragg Gratings); Transmissive WBGs; phase-shifted FBGs (withsingle or multiple passbands); Fabry-Perot (FP) micro-etalons andfilters; fiber FP filters and micro-optic; and micromachined FP filters;semiconductor FP filters; Micro-ring resonators; waveguide couplers;Interferometric-waveguide-based filters; Michelson Interferometric (MI)waveguide filters; Mach-Zehnder Interferometric (MZI) waveguide basedfilters; Arrayed Waveguide Gratings (AWGs); and polarizationinterferometric (PI) waveguide-based filters; fiber loop mirrors; andbulk-optic -based PI filters.
 29. The precisely wavelength-tunable laserrecited in claim 27, wherein said rapidly wavelength-tunable filter isselected from the group consisting of tunable Reflective Waveguide BraggGratings (WBGs); tunable FBGs (Fiber Bragg Gratings); tunable SWBG(Sampled Waveguide Bragg Gratings); tunable SFBGs (Sampled Fiber BraggGratings); tunable Transmissive WBGs; phase-shifted tunable FBGs (withsingle or multiple passbands); tunable Fabry-Perot (FP) micro-etalonsand filters; PZT-based fiber FP (FFP) filters; Liquid Crystal (LC)-basedfiber FP (FFP) filters; Microelectromechanical Systems (MEMS)-based FPfilters; electro-optic FP filters; current injection or optically tunedsemiconductor FP filter; tunable Micro-ring resonators; tunablewaveguide couplers; grating-assisted waveguide couplers andelectro-optic waveguide couplers; tunableInterferometric-waveguide-based filters; tunable MichelsonInterferometric (MI) waveguide filters; tunable Mach-ZehnderInterferometric (MZI) waveguide based filters; tunable Arrayed WaveguideGratings (AWGs); and tunable polarization interferometric (PI)waveguide-based filters; fiber loop mirrors with tunable electro-optic(EO) birefringent elements; fiber loop mirrors with tunablesemiconductor birefringent elements; and tunable bulk-optic-based PIfilters.
 30. The precisely wavelength-tunable laser recited in claim 27,wherein said gain medium is selected from the group consisting of: dopedfiber optic waveguides and semiconductor optical amplifiers.
 31. Theprecisely wavelength-tunable laser recited in claim 27, furthercomprising means for tuning said rapidly wavelength-tunable filter. 32.The precisely wavelength-tunable laser recited in claim 31, wherein saidmeans for tuning said rapidly wavelength-tunable filter is selected fromthe group consisting of: electro-optic tuning devices, strain-tunedtuning devices and electro-absorptive tuning devices.
 33. The preciselywavelength-tunable laser recited in claim 27, wherein said rapidlywavelength-tunable filter has a tuning speed of less than 50 μs.
 34. Theprecisely wavelength-tunable laser recited in claim 27, furthercomprising an optical coupling feedback device for combining opticalfeedback from said transmissive filter loop and said at least a firstarm.
 35. The precisely wavelength-tunable laser recited in claim 34,wherein said optical coupling feedback device is selected from the groupconsisting of: N-port optical circulators, fiber fused couplers, andwaveguide-Y junctions.
 36. A precisely wavelength-tunable laser, saidlaser comprising: a gain medium; a pump source, said pump sourceproviding light to said gain medium; a first laser cavity resonator,said pump source optically aligned with said first laser cavityresonator; a second laser cavity resonator, said gain medium disposed ineither said first laser cavity resonator or said second laser cavityresonator; an optical coupling feedback device for combining opticalfeedback from said first and second laser cavity resonators; a rapidlywavelength-tunable filter disposed in said first laser cavity resonator,wherein said wavelength-tunable filter is tunable by application ofvoltage across said wavelength-tunable filter; a multi-wavelength gridfilter disposed in said second laser cavity resonator; and means forremoving an output signal from one of said laser cavity resonators. 37.The precisely wavelength-tunable laser recited in claim 36, wherein saidmulti-wavelength grid filter is selected from the group consisting ofReflective Waveguide Bragg Gratings (WBGs); FBGs (Fiber Bragg Gratings);SWBG (Sampled Waveguide Bragg Gratings); SFBGs (Sampled Fiber BraggGratings); Transmissive WBGs; phase-shifted FBGs (with single ormultiple passbands); Fabry-Perot (FP) micro-etalons and filters; fiberFP filters and micro-optic; and micromachined FP filters; semiconductorFP filters; Micro-ring resonators; waveguide couplers;Interferometric-waveguide-based filters; Michelson Interferometric (MI)waveguide filters; Mach-Zehnder Interferometric (MZI) waveguide basedfilters; Arrayed Waveguide Gratings (AWGs); and polarizationinterferometric (PI) waveguide-based filters; fiber loop mirrors; andbulk-optic-based PI filters.
 38. The precisely wavelength-tunable laserrecited in claim 36, wherein said rapidly wavelength-tunable filter isselected from the group consisting of tunable Reflective Waveguide BraggGratings (WBGs); tunable FBGs (Fiber Bragg Gratings); tunable SWBG(Sampled Waveguide Bragg Gratings); tunable SFBGs (Sampled Fiber BraggGratings); tunable Transmissive WBGs; phase-shifted tunable FBGs (withsingle or multiple passbands); tunable Fabry-Perot (FP) micro-etalonsand filters; PZT-based fiber FP (FFP) filters; Liquid Crystal (LQ-basedfiber FP (FFP) filters; Microelectromechanical Systems (MEMS)-based FPfilters; electro-optic FP filters; current injection or optically tunedsemiconductor FP filter; tunable Micro-ring resonators; tunablewaveguide couplers; grating-assisted waveguide couplers andelectro-optic waveguide couplers; tunableInterferometric-waveguide-based filters; tunable MichelsonInterferometric (MI) waveguide filters; tunable Mach-ZehnderInterferometric (MZI) waveguide based filters; tunable Arrayed WaveguideGratings (AWGs); and tunable polarization interferometric (PI)waveguide-based filters; fiber loop mirrors with tunable electro-optic(EO) birefringent elements; fiber loop mirrors with tunablesemiconductor birefringent elements; and tunable bulk-optic-based PIfilters.
 39. The precisely wavelength-tunable laser recited in claim 36,wherein said gain medium is selected from the group consisting of: dopedfiber optic waveguides and semiconductor optical amplifiers.
 40. Theprecisely wavelength-tunable laser recited in claim 36, wherein saidrapidly wavelength-tunable filter has a tuning speed of less than 50 μs.41. The precisely wavelength-tunable laser recited in claim 36, whereinsaid optical feedback coupling device is selected from the groupconsisting of: N-port optical circulators, fused fiber couplers, andwaveguide-Y junctions.
 42. A precisely wavelength tunable laser, saidlaser comprising: a gain medium; a pump source, said pump sourceproviding light to said gain medium; a laser cavity resonator, said gainmedium disposed in said laser cavity resonator and said pump sourceoptically aligned with said laser cavity resonator; a rapidlywavelength-tunable filter disposed in said laser cavity resonator,wherein said wavelength-tunable filter is tunable by application ofvoltage across said wavelength-tunable filter; a multi-wavelength gridfilter disposed in said laser cavity resonator; a coupling means; andmeans for removing an output signal from said laser cavity resonator;wherein said laser cavity resonator has a transmissive filter loop andat least a first arm; said rapidly wavelength-tunable filter disposed insaid transmissive filter loop; and said multi-wavelength grid filterdisposed in said first arm, said transmissive filter loop coupled bysaid coupling means to said first arm.
 43. The precisely wavelengthtunable laser recited in claim 42, wherein said multi-wavelength gridfilter is selected from the group consisting of Reflective WaveguideBragg Gratings (WBGs); FBGs (Fiber Bragg Gratings); SWBG (SampledWaveguide Bragg Gratings); SFBGs (Sampled Fiber Bragg Gratings);Transmissive WBGs; phase-shifted FBGs (with single or multiplepassbands); Fabry-Perot (FP) micro-etalons and filters; fiber FP filtersand micro-optic; and micromachined FP filters; semiconductor FP filters;Micro-ring resonators; waveguide couplers;Interferometric-waveguide-based filters; Michelson Interferometric (MI)waveguide filters; Mach-Zehnder Interferometric (MZI) waveguide basedfilters; Arrayed Waveguide Gratings (AWGs); and polarizationinterferometric (PI) waveguide-based filters; fiber loop mirrors; andbulk-optic-based PI filters.
 44. The precisely wavelength tunable laserrecited in claim 42, wherein said rapidly wavelength-tunable filter isselected from the group consisting of tunable Reflective Waveguide BraggGratings (WBGs); tunable FBGs (Fiber Bragg Gratings); tunable SWBG(Sampled Waveguide Bragg Gratings); tunable SFBGs (Sampled Fiber BraggGratings); tunable Transmissive WBGs; phase-shifted tunable FBGs (withsingle or multiple passbands); tunable Fabry-Perot (FP) micro-etalonsand filters; PZT-based fiber FP (FFP) filters; Liquid Crystal (LC)-basedfiber FP (FFP) filters; Microelectromechanical Systems (MEMS)-based FPfilters; electro-optic FP filters; current injection or optically tunedsemiconductor FP filter; tunable Micro-ring resonators; tunablewaveguide couplers; grating-assisted waveguide couplers andelectro-optic waveguide couplers; tunableInterferometric-waveguide-based filters; tunable MichelsonInterferometric (MI) waveguide filters; tunable Mach-ZehnderInterferometric (MZI) waveguide based filters; tunable Arrayed WaveguideGratings (AWGs); and tunable polarization interferometric (PI)waveguide-based filters; fiber loop mirrors with tunable electro-optic(EO) birefringent elements; fiber loop mirrors with tunablesemiconductor birefringent elements; and tunable bulk-optic-based PIfilters.
 45. The precisely wavelength tunable laser recited in claim 42,wherein said gain medium is selected from the group consisting of: dopedfiber optic waveguides and semiconductor optical amplifiers.
 46. Theprecisely wavelength tunable laser recited in claim 42, furthercomprising means for tuning said rapidly wavelength-tunable filter. 47.The precisely wavelength-tunable laser recited in claim 42, furthercomprising a second arm in said laser cavity resonator, and second armcoupled to said first arm and said transmissive filter loop by saidcoupling means.
 48. The precisely wavelength-tunable laser recited inclaim 47, further comprising a mirror disposed at an end of said secondarm distal from said transmissive filter loop.
 49. The preciselywavelength-tunable laser recited in claim 42, wherein said rapidlywavelength-tunable filter has a tuning speed of less than 50 μs.